On a typical installation the area is edged with tanalised timber to keep the base secure. This is usually when the grass is being installed on open ground with the border defined by soil beds. Sometimes the team will encounter lawns that already have their borders well defined like this one in Surrey, or come up against paving or patios. In this case the lawn is edged with cement so that it secures both the pre-existing edging and creates a stable surface to fix the grass. An advantage with cement is that it follows the course more exactly, which makes it easier when there are curves involved.
You can see the team have done a bit of brickwork to complete the edging in one corner to make sure the completed lawn looks tidier. The usual layers of aggregate and weed membrane have also been put down but are not pictured.